Technical Field
The present disclosure relates generally to the field of combustion burners and methods of use, and more specifically to burners, submerged combustion melters, and methods of their use, particularly for melting glass-forming materials.
Background Art
A submerged combustion melter (SCM) may be employed to melt glass batch and/or waste glass materials to produce molten glass by passing oxygen, oxygen-enriched mixtures, or air along with a liquid, gaseous and/or particulate fuel (some of which may be in the glass-forming materials), directly into a molten pool of glass, usually through burners submerged in a glass melt pool. The introduction of high flow rates of products of combustion of the oxidant and fuel into the molten glass, and the expansion of the gases during submerged combustion (SC), cause rapid melting of the glass batch and much turbulence.
In the context of SCMs, known oxy-fuel burners are predominately water-cooled, nozzle mix designs and avoid premixing for safety reasons due to the increased reactivity of using oxygen as the oxidant versus air. One currently used submerged combustion burner employs a smooth exterior surface, half-toroid metallic burner tip of the same or similar material as the remainder of the burner. When operating an SCM with burners of this nature, the combustion burner tips are exposed to a glass and combustion gas environment high and oscillating temperatures. The burner can be designed so that the oxidant (typically oxygen) or fuel flow cools the inner wall of the burner tip before combustion occurs, but the flow cannot provide cooling to the top crown and outer wall of the burner tip. Although cooling water or other coolant is typically applied to cool the burner tip, the temperatures are extremely high for typical metal alloys when the hot glass (2500+° F. (1,370+° C.)) or combustion products (up to 4000° F. (2,190° C.) contact the burner tip. Also, since the burner tip section temperature is oscillating with the combustion bubble growth, the burner tip experiences frequent rapid temperature change. Therefore, burner life can be very short due to the failure of the materials used to form the burner tip.
Failure analysis of used SCM burner tips identified thermal fatigue cracking and hot spot formation in the toroid burner tip as two important modes of burner failure. Hot spots may form during the operation, likely due to partial plugging of the burner, which may direct combustion flame toward one side of the burner and cause localized heating or hot spot formation. This situation leads to rapid localized oxidation of the burner tip material, and consequent reduction in wall thickness of the burner tip. Thermal fatigue may be problematic because, for the most part, portions of the burner tip remain exposed to turbulent molten glass, while other portions are exposed to gaseous combustion mixture (for example natural gas and oxygen). Furthermore, while the burner tip is exposed to rapidly fluctuating temperature and loading conditions, lower portions of the burner tip, and the burner body below the burner tip, are immobilized by the refractory liner and steel shell of the melter. Being confined, the burner tip experiences a repetitive thermal stress pattern. Additionally, temperature gradients across the burner tip wall thickness cause thermal stresses at the apex of the burner tip crown. It is also known that film boiling occurs at the internal water-cooled surface of the burner tip (U-turn) causing rapid fluctuations in the thermal gradient across the burner wall thickness at the toroid shaped tip. It is believed this challenging service condition causes thermal fatigue failure of the burner by forming multiple and mostly radial cracks at the burner tip.
The hot spot formation can be addressed by suitably increasing the wall thickness. However, addressing the thermal fatigue issue (caused by temperature gradient dependent thermal stress) requires reduced wall thickness. While lower wall thickness (with reduced thermal gradient) extends fatigue crack initiation life, higher wall thickness favors crack propagation life. These contradictory requirements make it extremely challenging to design future burners for superior service life.
Development of submerged combustion burners having longer life and less susceptibility to the SCM environment while melting glass-forming materials would be a significant advance in the submerged combustion art.